This invention relates to systems and methods for monitoring the performance of a catalytic converter in diesel engines, and more particularly to the use of a carbon monoxide sensor to achieve this goal.
Vehicles equipped with diesel engines offer ecological and economical advantages compared to those equipped with conventional gasoline engines due to their higher fuel economy benefit. However, both diesel and gasoline engines discharge pollutants, such as carbon monoxide (CO), hydrocarbons (HC), nitrogen oxides (NOx), and particulate matter. Catalytic converters are used to reduce pollutants. These devices are typically installed in the exhaust system of the engine. As the catalytic converter ages, its ability to reduce combustion by-products, i.e. its conversion efficiency, diminishes.
Government agencies are requiring vehicles to be equipped with emission monitoring systems, commonly known as On-Board Diagnostic (OBD) systems, in order to inform the operator of the vehicle when the emissions exceed government allowed standards. It is required that the malfunction indicator light be turned on when levels of specific emissions exceed the standard by a designated factor. Current OBD requirements are targeted primarily at HC and NOx emissions. Therefore, in order to comply with the OBD requirements, it is important to monitor the performance of the catalytic converter.
One method of determining degradation in catalytic converter efficiency is described in U.S. Pat. No. 5,822,979. A sensor providing a signal indicative of the HC concentration of the exhaust gas is positioned downstream of the catalyst. The amount of energy that is transferred from the exhaust gas to the catalyst to achieve light-off is then calculated based on engine operating conditions. Light-off in this instance is defined as the point at which catalyst efficiency surpasses 50% for HC reduction. If the total energy input into the catalyst to achieve light-off is higher than a predetermined threshold value, it is inferred that the catalyst has deteriorated sufficiently to produce emission levels higher than allowed.
The inventors herein have recognized two disadvantages with this approach for vehicles equipped with diesel engines. First, monitoring HC emissions directly would not work on diesel engines. Since diesel emissions contain heavier, more condensable HC components than gasoline engine emissions, the heavy HC emissions can adsorb onto the catalyst at temperatures below light-off. Therefore, if HC levels are measured pre and post catalyst for diesel, it may appear that HC conversion is taking place when, in effect, the post catalyst HC reduction is due to it being stored on the catalyst. Thus, it is not possible to estimate catalyst efficiency in a diesel engine accurately by monitoring HC post catalyst. Second, catalysts on gasoline engines generally accumulate energy continuously as before and during light-off. They reach light-off temperatures in a short time after cold start. This is due to combination of high exhaust temperature and high levels of CO and HC. The latter produce enough of an exotherm to increase catalyst temperature for light-off, and maintain catalyst temperature well above light-off. In contrast, diesel catalysts usually dissipate much of the energy they adsorb before and after light-off. This is because of both low exhaust temperature and low levels of CO and HC. Diesel catalysts typically take much longer to achieve cold start light-off temperature than gasoline catalysts. Diesel catalysts will pass in and out of light-off, or partial light-off, in conjunction with accelerations and decelerations of the driving pattern. Therefore, using cumulative energy input to a catalyst to achieve light-off as a measure of performance will not work on diesel applications.
An object of the present invention is to provide a method and a system for improved monitoring of catalytic converter performance in vehicles equipped with diesel engines.
The above object is achieved and disadvantages of prior approaches overcome by a method for evaluating a performance of a catalytic converter, the converter coupled to a first sensor providing a first signal indicative of a catalytic converter temperature and coupled to a second sensor providing a second signal indicative of a concentration of an exhaust constituent exiting the catalytic converter, the method including the steps of: estimating a concentration of the exhaust constituent entering the catalytic converter based on operating conditions; calculating a conversion efficiency of the exhaust constituent based on said estimating and the second signal; creating a first table of said conversion efficiency versus the catalytic converter temperature; and determining whether the catalytic converter is operating below a predetermined efficiency level based on a mathematical comparison of said first table to a second table indicative of a threshold catalytic converter efficiency.
In an alternative embodiment, the above object is achieved and disadvantages of prior approaches overcome by a method for evaluating a performance of a catalytic converter in an internal combustion engine, the converter coupled to a first sensor providing a first signal indicative of a concentration of an exhaust constituent entering the catalytic converter and coupled to a second sensor providing a second signal indicative of a concentration of the exhaust constituent exiting the catalytic converter and coupled to a third sensor providing a third signal indicative of a catalytic converter temperature, the method including the steps of: calculating a conversion efficiency of the exhaust constituent based on the first and second signals; creating a first table of said conversion efficiency versus the catalytic converter temperature; and determining whether the catalytic converter is operating below a predetermined efficiency level based on a mathematical comparison of said first table to a second table indicative of a threshold catalytic converter efficiency.
In yet another aspect of the present invention, the above object is achieved an disadvantages of prior approaches overcome by a system for monitoring a performance of a catalytic converter with respect to a first exhaust constituent, the system consisting of: a diesel engine; a sensor providing a first signal indicative of a concentration of a second exhaust constituent entering the catalytic converter; a sensor providing a second signal indicative of a concentration of said second exhaust constituent exiting the catalytic converter; a sensor providing a third signal indicative of a catalytic converter temperature; and a controller for calculating a conversion efficiency of said second exhaust constituent based on said first and said second signals, determining a conversion efficiency of the first exhaust constituent based on a prestored table of the first exhaust constituent conversion efficiency vs. said second exhaust constituent conversion efficiency, and determining whether the catalytic converter is operating below a predetermined efficiency level with respect to the first exhaust constituent conversion efficiency.
An advantage of the above aspects of invention is that a more accurate estimate of catalytic converter performance in diesel engines can be achieved by monitoring pre and post catalyst carbon monoxide concentration rather than hydrocarbon concentration. Since current OBD requirements are targeted to HC and NOx emissions, the converter""s performance with respect to CO can then be correlated to that for HC and NOx. Also, the proposed method builds a carbon monoxide conversion rate versus temperature curve and compares it to a reference curve to evaluate catalyst performance. This method is more accurate than the prior art method which compares a single point performance to a single point on a reference curve. Having a more accurate estimate of the catalytic converter performance in diesel engines will allow stricter compliance to OBD requirements.
Other objects, features and advantages of the present invention will be readily appreciated by the reader of this specification.